The PR interval on a standard 12-lead ECG represents conduction through which cardiac structures in sequence?

Explanation

## PR Interval Definition The PR interval is measured from the **onset of the P wave to the onset of the QRS complex**. ### Anatomical Pathway **Key Point:** The PR interval encompasses conduction through the following structures **in sequence**: 1. **Atrial muscle** — represented by the P wave; impulse spreads from the SA node across both atria 2. **AV node** — the major site of physiological conduction delay (~100 ms) 3. **His bundle** — rapid conduction through the common bundle 4. **Purkinje fibres** — rapid distribution to ventricular myocardium (the QRS begins as Purkinje fibres activate ventricular muscle) > **Why not the SA node?** The SA node fires *before* the P wave begins; it is not included in the PR interval measurement, which starts at the **onset of the P wave** (i.e., onset of atrial depolarisation). > **Why include Purkinje fibres?** The QRS complex begins only when ventricular *myocardial* cells start to depolarise. The His–Purkinje conduction itself occurs within the PR interval (isoelectric segment after the P wave), so Purkinje fibre conduction is correctly included in the PR interval. ### Normal Duration **High-Yield:** Normal PR interval = **0.12–0.20 seconds (3–5 small squares)** on standard ECG paper (25 mm/s). ### Why Option B (SA node → atrial muscle → AV node → His bundle) is incorrect The PR interval is measured from the **start of the P wave**, not from SA node firing. The SA node depolarisation is electrically silent on surface ECG and precedes the P wave; it is therefore **not part of the PR interval**. Additionally, option B omits the Purkinje fibres, which conduct the impulse to the ventricular myocardium within the PR interval. ### Clinical Significance | Finding | Interpretation | |---------|----------------| | Prolonged PR (>0.20 s) | First-degree AV block | | Short PR (<0.12 s) | Pre-excitation (e.g., WPW syndrome) | | Progressive PR lengthening | Mobitz type I (Wenckebach) second-degree AV block | **Clinical Pearl:** The AV node accounts for approximately 100 ms of the total PR duration and is the primary site assessed when the PR interval is prolonged. [cite: Harrison 21e Ch 269; Guyton & Hall Medical Physiology 14e Ch 13]